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The sailors were traveling through the Persian Gulf from Kuwait toward Bahrain when U.S. controllers lost contact with them Tuesday. Iranian Revolutionary Guards who patrol the Gulf boarded the U.S. vessels and detained the crew members near Farsi Island, Iranian territory roughly midway between Kuwait and Bahrain.

Questions remain

A U.S. defense official told VOA that Navy tracking equipment found the sailors' boats to be "dead in the water, and in Iranian water."

"Why?" he asked. "We're still figuring that out."

The Navy then "found the boats on Farsi Island, parked with no crew."

As to whether it was mechanical failure or navigational error that put the sailors in Iranian territorial waters, the official said, "Let the investigation work itself out."

A U.S. defense official added, "We do know that it likely wasn't mechanical failure. There's a chance that it was navigational error instead, but we really must await completion of the ongoing investigation into the circumstances."

U.S. sailors under detention in the Farsi Island by Iran's Revolutionary Guards after investigations showed their patrol boats had entered Iranian waters unintentionally, Jan. 13, 2016.

"The [Obama] administration is pretending as if nothing out of the ordinary has occurred," Republican Senator John McCain of Arizona said in a statement, pointing out that under international law, navy boats do not lose their sovereign immune status when they are in distress at sea.

"By failing to affirm basic principles of international law, it places our Navy and Coast Guard vessels and the men and women who sail them at increased risk in the future," McCain said.

Peaceful resolution

Kerry praised Iran for "swiftly resolving" the situation.

"That this issue was resolved peacefully and efficiently is a testament to the critical role diplomacy plays in keeping our country safe, secure and strong," Kerry said.

President Barack Obama delivers his State of the Union address before a joint session of Congress on Capitol Hill in Washington, Jan. 12, 2016.

The State Department said there was no formal U.S. apology to Iran, since the sailors' intrusion into Iranian waters was accidental.

Iranian television broadcast what appeared to be a brief interview with one of the American sailors, who said the crew apologized for entering Iranian waters by mistake.

Iranian Foreign Minister Mohammed Javad Zarif tweeted that he was happy to see "dialogue and respect, not threats and impetuousness, swiftly resolved the sailors episode. Let’s learn from this latest example."

Ramadhan Sharif, a spokesman for the Revolutionary Guards, told Iran’s Aftab News that the U.S. servicemen were questioned while in detention.

The situation in the Gulf was unfolding as Obama went to Capitol Hill Tuesday evening to deliver his annual State of the Union message, televised live through the country and abroad. The president did not mention the Gulf incident, but did take note of the nuclear agreement reached last year between Tehran and a group of world powers, saying, "The world has avoided another war."

The nuclear agreement is expected to be implemented in the coming days, following steps Iran has taken to curb its nuclear activities. Western governments have agreed in return to lift long-standing economic sanctions against the Islamic Republic.

Farsi Island is home to an Iranian Revolutionary Guard Corps naval base, which may be why the sailors were quickly detained, according to Matthew Kroenig, a senior fellow in The Atlantic Council’s Brent Scowcroft Center on International Security.

“Most countries would do the same thing if foreign sailors came that close to a naval base,” Kroenig told VOA.

Less than a month ago U.S. officials criticized Iran for carrying out a "highly provocative" rocket test near U.S. boats passing through the Strait of Hormuz in the region.

Chris Hannas in Washington, Nike Ching and Pam Dockins at the State Department, and other VOA journalists contributed to this report.